1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having a lead plate attached thereto.
2. Description of the Prior Art
Recently, secondary batteries have been developed and used extensively because they are rechargeable and have the potential to be compact, yet have a large capacity. Typical examples of secondary batteries include nickel-metal hydride (Ni-MH) batteries, lithium (Li) batteries, and lithium ion batteries.
In most cases, the bare cell of secondary batteries is formed by placing an electrode assembly, composed of positive and negative electrodes and a separator, into a can which is made of iron, aluminum, or aluminum alloy, covering the can with a cap assembly, injecting an electrolyte into the can, and sealing the cap assembly. If the can is made of aluminum or an aluminum alloy, the weight of the batteries may be advantageously reduced because aluminum is lightweight. In addition, the batteries do not become corroded even when they are used for a long time under high voltage.
In general, the bare cell of secondary batteries is provided with an electrode terminal on its upper portion. The electrode terminal is insulated from its surroundings and is connected to an electrode inside the bare cell to form the positive or negative terminal of the battery. The can itself has a polarity opposite to that of the electrode terminal.
The electrode terminal of a sealed bare cell of a secondary battery is electrically connected to a terminal of a safety apparatus, such as a positive temperature coefficient (PTC) device or a protective circuit module (PCM). The safety apparatus is connected to positive and negative terminals and prevents any danger, such as fracture of the battery, by interrupting the current when the temperature of the battery rises drastically or the voltage increases abruptly due to, for example, overcharging or over-discharging.
Generally, it is difficult to electrically connect the electrodes of the bare cell to the electric terminals of, for example, a PCM, by direct welding because of the shape and the material of the bare cell. Accordingly, a conductor structure, called a “lead plate,” is used to connect the positive and negative electrodes of the battery to the electric terminals of a safety apparatus, e.g., a PCM. The lead plate is usually made of nickel, a nickel alloy, or nickel-plated stainless steel. The safety apparatus and the bare cell are placed into a separate pack where they are electrically connected to each other. Alternatively, a melt resin may be used to fill and coat the space between the safety apparatus and the bare cell to complete the battery pack.
However, a problem may occur when trying to weld a lead plate made of nickel to a can made of aluminum. Because of the high melting points of nickel and aluminum, and the excellent conductivity of aluminum, it is very difficult to successfully use ultra-sonic welding or resistance welding on these materials. Therefore, a laser is generally used to weld the can to the lead plate. If such laser welding is performed while the lead plate is connected to a protective circuit, the irradiating laser beams may result in potential electric shock or present other safety hazards. According to a conventional method, the lead plate is first welded to a can-type battery, and then the terminal plate of the protective circuit side is welded to the lead plate by resistance welding.
Further, when the lead plate is directly welded to the can, and specifically, to the bottom surface of the can, by laser welding, the electrolyte may leak from the welded portion if the welding strength is not correctly controlled. This is because the can has a thickness of about 0.2 to 0.3 mm, according to the typical method of making batteries in a flat shape with reduced weight. Therefore, the lead plate is, in many cases, formed on a part of the cap assembly of the can-type battery, usually on the cap plate.
When the lead plate is connected to the cap plate, the bare cell and the PCM are, in many cases, retained in a mold for a molding resin while they are connected to each other by the lead plate welding, and the gap is filled with molding resin to complete a resin molding type secondary battery. Such a resin molding type secondary battery is advantageous in that it has a smooth appearance as compared to the case where a separate case for a hard pack is used.
FIG. 1 shows a schematic lateral sectional view of the upper portion of a bare cell illustrating the problem occurring when a lead plate is welded to a side of a cap plate of a secondary battery according to the prior art. An electrode assembly 12, which is formed by laminating and winding negative and positive electrodes 15 and 13 and a separator 14, is inserted into a can 11, and a cap assembly is coupled to the open upper portion of the can. The cap assembly has a cap plate 110 as a main body and a negative terminal 130 formed in the central hole 113 of the cap plate 110 via an insulating gasket 120. The cap plate 110 has an electrolyte injection hole 112 formed on a side thereof adjacent the negative terminal 130. The cap plate 110 may also have a safety vent (not shown) positioned on the other side of the negative terminal 130. The electrolyte injection hole allows an electrolyte to be injected into the can 11 after the can has been covered with the cap assembly. After the electrolyte has been injected, the electrolyte injection hole 112 is sealed by a plug 160, which is formed by press-fitting an aluminum ball into the electrolyte injection hole.
However, in a conventional resin molding type secondary battery wherein the plug 160 is formed by press-fitting an aluminum ball into the electrolyte injection hole formed on the cap plate, a minute gap is likely to exist between the electrolyte injection hole 112 and the plug 160. As a result, laser welding is performed between the plug and the cap plate around the plug in order to prevent the electrolyte from leaking through the gap. It is also possible to prevent the leakage of the electrolyte by a two-step process of applying a liquid resin (or resin droplets) to the plug 160 and curing it by light or heat to form a resin plugging member 250.
The resin plugging member 250 or the plug 160 inevitably protrude out of a surface of the cap plate as a result of the method of forming them. A lead plate 210 has a bottom portion 211 having a predetermined area for surface-to-surface coupling with the cap plate 110 of the bare cell and a wall portion 213 protruding vertically toward the PCM from the bottom portion 211 for coupling with the electric terminal of the PCM. At least a part of the lead plate 210 is superimposed on the electrolyte injection hole 112. When welding is performed to couple the bottom portion 211 of the lead plate to the cap plate 110, the plug 160 or the resin plugging member 250, which protrudes out of the electrolyte injection hole 112, causes the bottom portion 211 of the lead plate to float on the cap plate 110, as shown schematically in FIG. 1. This protrusion interferes with the welding and may cause the weld to be weakened.
The lead plate 210 acts as a conducting path for connecting the cap plate, which is the positive terminal of the bare cell, to the connection terminal of the PCM. The lead plate 210 is inserted into the molding resin portion, which couples the PCM and the bare cell to each other in the resin molding type secondary battery, to firmly retain the bare cell. If welding fails to be correctly performed between the lead plate and the cap plate, the lead plate cannot accomplish the above functions. As a result, the mechanical strength or the electric connection of a finished secondary battery deteriorates. Therefore, there is a need for a secondary battery that addresses the above-described problems.